1994 Buick Regal with 3.8L Using Actron CP9150 code reader.

My sons car has an intermittent misfire on 2&5 but it occurs at odd times. 2 times it has happened when he parked the car on a down hill slope with one side of the front end dipped down at an angle. We live in the country and there is a * curve in one of the roads about a1.5 miles our house. 3 times when going around the left curve, it caused a misfire. These misfires do not go away immediately and my son drove the car home with the misfire each time.
Once when he was leaving the house the car was misfiring and when he went around the * curve to the right, the misfire stopped. He also said that it will sometimes start to misfire when he comes to a stop and starts back off again. The code is a P0300. When the engine is not running in a constant misfire on 2&5, the code reader live data shows very short random misfires on 1,2,3, and 4 while idling that do not set off a code. If we turn the AC on while idling those short random misfires go away. I know when you turn the AC on that the engine RPM'* raise a little.

We swapped coil 2&5 with coil 1&4 and cleared the codes. My son drove close to 100 miles before the engine started to misfire again. This time it started to misfire after he went through the * curve to the left. When he got home he connected the code reader and it was showing 2&5 to be misfiring.

I think we have had the car almost 3 years now and when we first bought the car we replaced the head gaskets and upper intake. We also put in new plugs and wires. My son has only driven the car about 10k since then so the wires and plugs should be good.

Someone before we bought the car has replaced the ICM connector because all of the wires are white. All of the splices must be tucked back inside the sheath covering the wires because none of the connectors are visible.

We do have another 1994 Buick regal with a 3.8 for a spare parts car. We test drove the parts car before buying it and ran the codes and it had a bad TPS. WE were going to fix the car but then some rats chewed some wiring and the fuel line so it is a parts car now. We could try swapping what ever you guys recommend we try..

Sorry for the post being so long but was wanting to include as much info as I could and thanks in advance for your help!
Anna

Is this misfire fuel level dependent? Will it do it on a full tank of gas as compared to a nearly empty one?

Does the misfire ever go away, like, say, if the vehicle is restarted, or cold started again?

Why was the head gasket and upper intake first replaced at purchase?

Thanks for the reply Rjolly87.
No it is not fuel level dependent though there is a rubber hose from the fuel tank that looks pretty deteriorated but I think it has to do with the evap system and there are no codes for the evap at this time.

Yes the 2-5 misfire goes away sometimes after the engine is started the next day. Last night it started the misfire just before he got home and I just started the engine and the misfire is still there this afternoon. Interesting this time is that when I turned the key to the run position, without trying to start the engine, the fuel pump did not shut off. I turned the key to the off position and right back to the run position and the fuel pump did not come back on till after I started the engine. I cut the engine off and restarted it several times and revved the engine a bit but the misfire did not go away however the fuel pump is working correctly now. It runs for a few seconds and then cuts off.

AH, My son changed the fuel filter a couple of days ago. The car had not been started for the day and there was no pressure at the shrader valve when he went to release the pressure.

Shortly after buying the car we noticed after the car had been driven and was up to temp and cut off for 10 to 20 minutes, it had a rough idle for a minute or so after restarting the engine. We used one of the chemical tests to check for a bad head gasket and the liquid quickly turned green indicating a bad head gasket. Also the radiator was clear full of fer sticky gunk and the engine had a new water pump so we were guessing that the PO had ran it hot...

Thanks
Anna

I suspect a bad ICM, but would like to know the fuel pressure when it is doing this.

I guess it is time to invest in a fuel pressure gauge. None of the parts stores around here rent one. Are the HF ones any good? Looking online NAPA and AZ sell the same gauge for around $44.00 and has TEC on the front. We did listen to the injectors with a mechanics stethoscope and I was surprised at how much noise they make. A couple were louder then others but we didn't write them down. We can double check that tomorrow. Right after we put the engine back together, one of the injectors was clicking but it quieted down within a couple of days.

If it doesn't rain tomorrow maybe we can try swapping some oils and the ICM with the parts car. I read online that the parts stores can test the ICM. If it is not malfunctioning at the time of the test then that would be pointless perhaps?

thanks
Anna

Something else I forgot to add. We checked the vacuum during the misfire and it was at 16. When the engine was not misfiring, the vacuum was 22. I would guess that is normal if 2 cylinders are misfiring. Also the engine shakes a little and you can feel it in the car and hear the misfire in the exhaust.

I would do a wiggle test of the wiring for the ignition under hood and pull on the harnesses and see if anything causes it to misfire. If not I would test the ICM.

Yeah, supporting an older car it would be a good idea to have a fuel pressure gauge. You don't need a super high quality one for occasional use though.

Ok,, So now I am confused, more...

Fuel pressure is 38psi running and 46psi when you take the vacuum line off of the FPR. With the engine off, the fuel pressure did not drop in the 10 or 15 minutes the were working on it without starting the engine again.

Did the wiggle test and no change...

So the code reader says that 2&5 has a constant misfire. In my original post I think I mentioned that we tried swapping the 2&5 coil with 1&4. When we started the engine back up, it was no longer misfiring but did start misfiring with in 100 miles. The misfire has not gone away since and the car has not been driven out of the driveway.

Today we swapped coil pack 2&5 with 3&6 and nothing changed. The code reader is still saying 2&5 are misfiring. SO,,, I removed the injector connectors one at a time and guess what? 1&4 make absolutely no change in the engine while 2&5 and 3&6 do with the engine almost dying when I disconnected an injector wire.

I used DVM on the injector 1, 4, and 3 to check the ohms with the plugs disconnected and the readings were 1 was 15.2 while 4 read 14.8, and cylinder 3 injector read 14.8 ohms..

We used a test light and to check for spark at coil pack 1&4. The alligator clip for the test light was on a grounded body part and the tip of the test light was placed next to plug wire boot at coil 1 while we pulled the boot off. There was no spark at all. We did the same with wire 4 and no spark. To make sure the tester was working we did the same with #2 on coil pack 2&5. With the tip of the test light about an 8th inch away, there was a orange spark but as my son pulled the tester tip farther away, the spark turned blue. At about 1" away there was a tiny bit of orange at the tip of the coil stud and tester tip with almost an inch of blue in the middle.

Next we swapped coil pack 1&4 with the parts car and it produced no spark either. So that means for some reason the ICM is not sending signal to the 1&4 coil would be my guess as a couple of you guys have already mentioned!

I listened to all 6 injectors with a stethoscope and all of them were clicking.

The parts car still has the ICM and PCM. and the odometer is showing 60k

I guess the next step is to test the ICM
Thanks
Anna

A couple of more questions please.

does the ICM control which and when each coil fires? Can the ECM cause one of the coils not to fire? The ICM also controls the injectors?
I mentioned that the PO had replaced the ICM connector and so all of the wires are white and the splices must be inside the sheath because I cant see any of them. The parts car though, it has the original connector and wire colors. and it looks like all fo the injector wires go into the ICM plug.

Thanks
Anna